Coordinated control method of distribution network voltage based on OLTC and energy storage system

Abstract

The present invention belongs to the technical field of power system stability control and relates to a method for coordinated voltage control of a distribution network based on OLTC and an energy storage system. First, a distribution network model including OLTC, an energy storage system, and a photovoltaic system is established. Then, the weighted average voltage of the distribution network is calculated using the node voltage deviation and the state of charge deviation of the energy storage system, and fed back into the control signal of the OLTC. Two control modes are designed for the energy storage system based on the node voltage and the state of charge of the energy storage system: voltage control mode and SOC control mode. These modes fully utilize the rapid response capability of the energy storage system and achieve rapid control of the voltage at the access point of the photovoltaic system through active power regulation. The advantages of the two devices are fully utilized to achieve rapid voltage regulation of the distribution network while improving the cycle life.

Description

Technical Field

[0001] The present invention belongs to the technical field of power system stability control, and in particular relates to a distribution network voltage coordinated control method based on OLTC and energy storage system. Background Art

[0002] With the vigorous development and widespread application of photovoltaic power generation technology, the access capacity of distributed photovoltaic power generation systems in medium-voltage distribution networks has increased year by year. However, this rapid increase in photovoltaic capacity has adversely affected distribution network voltage control. On the one hand, photovoltaic systems inject large amounts of active power into the distribution network during peak power generation periods, causing reverse power flow and, in turn, voltage violations. On the other hand, due to the high impedance ratio of transmission lines, the randomness of photovoltaic output can also cause rapid and frequent voltage fluctuations at the access point, seriously affecting the voltage quality and power supply reliability of the distribution network. Therefore, it is of great significance to study coordinated voltage control technologies for distribution networks with a high proportion of photovoltaic power generation systems.

[0003] To address the voltage issues caused by high-proportion photovoltaic integration into distribution networks, traditional voltage control methods, namely on-load tap changers (OLTCs), are slow to respond and are limited by the number of switching cycles, making it difficult to immediately respond to the rapid and frequent voltage changes after photovoltaic integration. Therefore, deploying battery energy storage systems (BESS) at photovoltaic access points is considered an effective means of addressing voltage control issues in medium-voltage distribution networks due to their fast and flexible response characteristics and gradually decreasing costs. Taking into account the capacity and state of charge (SOC) of the energy storage device, active power regulation of the energy storage system can reduce peak loads and fill valleys, rapidly controlling voltage levels.

[0004] Existing voltage control methods ignore the coordination between energy storage systems and OLTCs, which, to a certain extent, reduces the overall voltage regulation capability of the distribution network. Therefore, this application proposes a distribution network voltage coordination control method based on OLTCs and energy storage systems, which effectively controls the voltage level while achieving rational utilization of resources within the distribution network. Summary of the Invention

[0005] In view of the shortcomings of the existing technology, the technical problem to be solved by the present invention is to propose a distribution network voltage coordinated control method based on OLTC and energy storage system, which effectively controls the voltage level while realizing the rational utilization of resources in the distribution network.

[0006] The technical solution adopted by the present invention to solve the technical problem is: a distribution network voltage coordinated control method based on OLTC and energy storage system, characterized by comprising the following steps:

[0007] Step 1: Establish a distribution network model that includes the OLTC, energy storage system, and photovoltaic system. The load at each node is of constant power factor type, the photovoltaic system adopts maximum power point tracking control, and the energy storage system adopts the PQ control strategy.

[0008] Step 2: Collect the active power and reactive power of each node in the distribution network, as well as the active power of each photovoltaic system. Calculate the node voltage of each node in the distribution network based on the distribution network power flow calculation method. Determine whether the node voltage exceeds the limit based on the node voltage. If the node voltage does not exceed the limit, repeat the node voltage calculation. If the node voltage exceeds the limit, use the communication system in the distribution network to collect the state of charge of all energy storage systems and calculate the weights and weighted voltage average of all node voltages in the distribution network.

[0009] Step 3: During the OLTC control process, the voltage error is determined by the deviation between the weighted voltage average and the OLTC secondary voltage reference value, and the required tap position adjustment is calculated based on the voltage error.

[0010] Step 4: The energy storage system controller includes two control modes;

[0011] Voltage control mode: When the node voltage of the energy storage system is lower than the minimum node voltage or higher than the maximum node voltage, the energy storage system operates in voltage control mode; the charging and discharging active power of the energy storage system is controlled by the PI controller, and the input of the PI controller is the node voltage deviation; the state of charge limiter is used to check whether the state of charge of the energy storage system exceeds its limit, and then the voltage control mode node considering the state of charge limit is obtained. i The energy storage system charging and discharging active power:

[0012] SOC control mode: When the node voltage is between the maximum and minimum limits, the energy storage system operates in SOC control mode. The charging and discharging active power of the energy storage system is controlled by the state of charge deviation.

[0013] Step 5: When the energy storage system's state of charge is within the limit, the node voltage deviation is adjusted by the energy storage system's charging and discharging active power. When the energy storage system's state of charge exceeds its upper and lower limits, the node voltage level cannot be maintained by providing the energy storage system's charging and discharging active power. At this time, the node voltage deviation and state of charge deviation both increase, thereby increasing the weight of the corresponding node. The corresponding node voltage level is adjusted by changing the OLTC tap position.

[0014] More specifically, the node voltage weight is calculated as follows:

[0015]

[0016] in, w i For nodesi The node voltage weight, ∆V i For nodes i The node voltage deviation, that is, the node i Node voltage V i With reference voltage V i，ref The difference, ∆SOC i For nodes i The state of charge deviation of the node i The state of charge of the energy storage system SOC i With node i Reference value of state of charge of energy storage system SOC i,ref The difference.

[0017] More specifically, the weighted average voltage of all nodes in the distribution network is calculated as follows:

[0018]

[0019] Where N is the number of nodes in the distribution network.

[0020] More specifically, the voltage error is calculated as follows:

[0021]

[0022] in, e is the voltage error, V ref is the OLTC secondary side voltage reference value, V avg is the weighted average voltage of all nodes in the distribution network.

[0023] More specifically, the voltage error is used to calculate the required adjustment tap position as follows:

[0024]

[0025] in, k Adjust the tap position as required ， V step is the voltage improvement value of each tap step of OLTC.

[0026] More specifically, the PI controller reference voltage is determined based on the node voltage V PI,ref :

[0027]

[0028] in, Vi For nodes i The node voltage, V min is the minimum node voltage, V max is the maximum node voltage.

[0029] More specifically, the voltage control mode node i The charging and discharging active power of the energy storage system is determined as follows:

[0030]

[0031] in, P i,V Node in voltage control mode i The energy storage system charging and discharging active power, P PI Output active power for PI controller; SOC min and SOC max are the minimum and maximum charge states of the energy storage system, SOC i For nodes i The state of charge of the energy storage system.

[0032] More specifically, the nodes in SOC control mode i The charging and discharging active power of the energy storage system is:

[0033]

[0034] in, P i,SOC is the charging and discharging active power of node i under SOC control mode, R is a constant, S t is the scaling factor, SOC i For nodes i The state of charge of the energy storage system, SOC i,ref For nodes i The charge state reference value of the energy storage system.

[0035] Compared with the prior art, the present invention has the following beneficial effects:

[0036] 1) The proposed coordinated voltage control strategy based on OLTC and energy storage systems uses voltage deviations and energy storage system SOC deviations to calculate node voltage weights in the distribution network. This strategy then calculates the weighted average voltage of the entire system and feeds it back into the OLTC control signal. This process achieves coordinated control of the OLTC and energy storage systems by assigning higher weights to nodes requiring immediate OLTC action.

[0037] 2) This invention designs two control modes for the energy storage system based on local node voltage information and the energy storage system's SOC status: voltage control mode and SOC control mode. This fully leverages the energy storage system's rapid response capability and achieves rapid voltage control at the photovoltaic system's access point through its active power regulation.

[0038] 3) The present invention coordinates energy storage and OLTC to improve the voltage level at distribution network nodes by sensing the node voltage deviation and the SOC value of the energy storage system, reducing the discharge depth of the energy storage system and improving the cycle life. At the same time, it avoids frequent tap adjustments of the OLTC, achieving good control effects and realizing the rational utilization of equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is the structure diagram of the IEEE 13-bus distribution network test system;

[0040] Figure 2 This is the OLTC control structure diagram;

[0041] Figure 3 It is the control structure diagram of the energy storage system;

[0042] Figure 4 It is the 24h load curve and photovoltaic output curve;

[0043] Figure 5 is a graph of the voltage at node 634;

[0044] Figure 6 It is the SOC curve diagram of the energy storage system at node 634;

[0045] Figure 7 It is the OLTC tap position change diagram within 24 hours;

[0046] Figure 8 It is the total energy absorbed / released by the energy storage system in 24 hours.

[0047] In the figure, BESS is the energy storage system, PV is the photovoltaic system, and 611, 632, 633, 634, 645, 646, 650, 652, 671, 675, 680, 684, and 692 are the node numbers. DETAILED DESCRIPTION

[0048] The technical solution of the present invention is described in detail below with reference to specific embodiments and drawings, but the scope of protection of the present application is not limited thereto.

[0049] The present invention provides a method for coordinated voltage control of a distribution network based on OLTC and an energy storage system, comprising the following steps:

[0050] Step 1: Establish a distribution network model including OLTC, energy storage system (BESS) and photovoltaic system (PV), where the load of each node is of constant power factor type, and the active power and reactive power are P d, 、 Q d, , the photovoltaic system (PV) adopts maximum power tracking control, and its active power output P g The energy storage system adopts PQ control strategy, and the active power and reactive power of the energy storage system are P BESS, 、 Q BESS , Q BESS Constantly 0.

[0051] This embodiment takes the improved IEEE 13-node distribution network as an example for explanation. Its topology is as follows: Figure 1 shown. Figure 1 In this example, an OLTC connects a 110 kV high-voltage grid to a 10 kV three-phase distribution network. PV systems and energy storage systems are installed at the ends of each of the four branches. The transmission line is an LGJ-50 overhead line with a unit impedance of (0.63 + j0.368) Ω / km and an impedance ratio of R / X = 1.7. Parameters for the PV and energy storage systems are shown in Table 1.

[0052]

[0053] Step 2: Calculate the weighted average voltage of each node:

[0054] Collect the active and reactive power of each node in the distribution network, as well as the active power of each photovoltaic system. Calculate the node voltage at each node using the distribution network power flow calculation method. Based on the node voltage, determine whether the node voltage exceeds the limit. According to GB∕T 33593-2017, "Technical Requirements for Grid-Connected Distributed Generation," the maximum allowable deviation of system node voltage is 5%.

[0055] (1)

[0056] in, V i For nodes i Node voltage, 1≤i ≤ N, N is the number of nodes in the distribution network, V min is the minimum node voltage, V max is the maximum node voltage, V min =0.95pu,V max =1.05pu.

[0057] If no node voltage exceeds the limit, the voltage calculation is repeated. When a node voltage exceeds the limit, the communication system in the distribution network is used to collect the charge status of all energy storage systems.

[0058] Equation (2) is used to calculate the voltage weights of all nodes in the distribution network to represent their priority in the voltage control process. The higher the voltage limit or the smaller the available state of charge capacity of the energy storage system at the node, the higher the assigned weight.

[0059] (2)

[0060] in, w i For nodes i The node voltage weight, ∆V i For nodes i The node voltage deviation, that is, the node i Node voltage V i With reference voltage V i，ref The difference, SOC i For nodes i The state of charge of the energy storage system, ∆SOC i For nodes i The state of charge deviation of the node i The state of charge of the energy storage system and the node i Reference value of state of charge of energy storage system SOC i,ref The difference, sign The symbol for the product of the two:

[0061] (3)

[0062] Calculate the weighted average voltage of all nodes in the distribution network V avg ;

[0063] (4)

[0064] Step 3: During OLTC control, the weighted voltage average value is used V avgand OLTC secondary side voltage reference value V ref The voltage error is determined by the deviation e :

[0065] (5)

[0066] Calculate the required adjustment tap position based on the voltage error k :

[0067] (6)

[0068] in, V step is the voltage improvement value of each tap step of OLTC. The OLTC controller of the proposed scheme is as follows Figure 2 As shown, P d,i For nodes i The active power, Q d,i For nodes i The reactive power, P g,i For nodes i The active power output of the photovoltaic system.

[0069] Step 4: The control structure of the energy storage system controller is as follows: Figure 3 As shown, V BESS is the output voltage of the energy storage system, P ref The output power reference value of the energy storage system; the energy storage system operates in active power control. Two control modes are designed for the energy storage system controller: voltage control mode and SOC control mode.

[0070] 1) Voltage control mode: When the node voltage of the energy storage system is lower than the minimum node voltage V min or higher than the maximum node voltage V max When , the energy storage system operates in voltage control mode; the PI controller (PI) is used to control the charging and discharging active power of the energy storage system. The input of the PI controller is the node i Node voltage V i With reference voltage V i，ref The difference (i.e., the node i Node voltage deviation ∆V i ); where the PI controller reference voltage is determined according to the node voltage V PI,ref :

[0071] (7)

[0072] Research shows that the depth of discharge of the energy storage system is inversely proportional to the expected average number of cycles of the state of charge (SOC). Therefore, in order to maximize the number of cycles of the energy storage system and improve the life of the energy storage system, the minimum state of charge of the energy storage system is determined. SOC min and the maximum state of charge of the energy storage system SOC max To limit the discharge depth of the energy storage system. When the energy storage system operates in voltage control mode, the SOC limiter is used to check whether the state of charge of the energy storage system exceeds its limit, and then the voltage control mode node considering the state of charge (SOC) limit is obtained. i The energy storage system charging and discharging active power:

[0073] (8)

[0074] in, P i,V Node in voltage control mode i The energy storage system charging and discharging active power, P PI Output active power for PI controller; SOC min and SOC max are the minimum and maximum charge states of the energy storage system, SOC i For nodes i The state of charge of the energy storage system.

[0075] In the above case, when the state of charge of the energy storage system at node i exceeds its upper and lower limits, the energy storage system cannot maintain the node voltage level of node i by providing active power. At this time, the node voltage deviation of node i is ∆ V i and SOC deviation ∆ SOC i The node voltage at node i is adjusted by changing the OLTC tap position. Once the state of charge of the energy storage system at node i reaches its limit, the power of the energy storage system at node i is cut off, thus avoiding overcharging and discharging of the energy storage system at node i.

[0076] 2) SOC control mode: When the node voltage at node i is between the maximum limit and the minimum limit, the energy storage system at node i operates in the SOC control mode. The charge and discharge active power of the energy storage system at node i is controlled by the state of charge deviation. The charge and discharge active power of the energy storage system at node i in the SOC control mode is:

[0077] (9)

[0078] in, P i,SOC is the charging and discharging active power of node i under SOC control mode, R is a constant determined by the thermal limit of the energy storage system, R Determines the maximum charge / discharge rate of the battery in SOC control mode; S t It is the proportional factor, which ensures that the energy storage system does not cause voltage rise / fall in SOC control mode; SOC i For nodes i The state of charge of the energy storage system, SOC i,ref For nodes i The charge state reference value of the energy storage system.

[0079] Step 5: OLTC and energy storage system coordinated control strategy: When the energy storage system's state of charge is within the limit range, the node voltage deviation is adjusted by the energy storage system's charging and discharging active power. When the energy storage system's state of charge exceeds its upper and lower limits, the node voltage level cannot be maintained by providing the energy storage system's charging and discharging active power. At this time, the node voltage deviation and state of charge deviation both increase, thereby increasing the weight of the corresponding node, and the corresponding node voltage level is adjusted by changing the OLTC tap position.

[0080] In MATLAB / Simulink, we built Figure 1 The distribution network system model shown in the figure was used to simulate and verify the voltage control strategy proposed in this paper over a 24-hour period. Both the data collection interval and simulation time scale were 1 minute, and transient voltage fluctuations caused by the operation of the OLTC and the battery energy storage system (BESS) were ignored. The upper and lower voltage limits were 0.95 pu to 1.05 pu, and the upper and lower limits of the battery energy storage system's state of charge (SOC) were 40% to 80%. The PV output and load curves for a portion of the distribution network are shown in Figure 4.

[0081] Figure 5 and Figure 6 They are the voltage curve of node 634 and the SOC curve of the energy storage unit under the two control strategies respectively. Figure 5The medium load curve and the output of the photovoltaic system are analyzed. From 6h to 12h, the photovoltaic output continues to increase, while the load is relatively small at this time. In the uncoordinated strategy, the energy storage system maintains the node voltage by absorbing energy. When the energy storage system SOC reaches the upper limit, some nodes begin to produce overvoltage, reaching a peak at around 12h at noon. Subsequently, the photovoltaic output gradually decreases and the user load slowly increases. At night, the user load reaches a peak, while the photovoltaic output is 0. The energy storage system maintains the node voltage by releasing energy. When the energy storage system SOC reaches the lower limit, some nodes begin to be undervoltage at 19h. The load continues to decrease until late at night, and the distribution network voltage returns to normal at around 23h. Using the control strategy of the present invention, it can be seen that there is only a brief voltage limit violation at 11h, and the energy storage system SOC always remains near the optimal value.

[0082] Figure 7 Figure 1 shows the OLTC tap position curves for the coordinated control method of the present invention and the uncoordinated strategy. Compared to the uncoordinated strategy, the present invention addresses voltage over-limit issues by adjusting the OLTC tap position multiple times, thus preventing excessive charging and discharging of the energy storage system. Although the number of OLTC tap operations per day increases from 4 to 10, it remains within the maximum allowed daily number of operations (typically 20).

[0083] In order to quantify and compare the utilization rate of the energy storage system between the coordinated control method of the present invention and the energy storage system without coordination strategy, the total energy absorbed / released by the energy storage system throughout the day (kW·h) is selected as the performance indicator. The results are shown in Figure 2. Figure 8 Compared to the uncoordinated strategy, the coordinated control method of the present invention significantly reduces the total energy absorbed and released by the energy storage system. This is because the changes in the OLTC taps in the present strategy bring the node voltage closer to the upper and lower limits, allowing the energy storage system to adjust the voltage level to within the allowable range by absorbing and releasing only a small amount of power. Furthermore, the control method of the present invention allows the energy storage system to operate at a low power level for most of the time, reducing the depth of discharge and thus improving the cycle life of the energy storage system.

[0084] The above analysis demonstrates that the coordinated control method proposed in this paper can effectively control node voltage levels within the distribution network, reducing the duration and magnitude of voltage overshoots, thereby facilitating voltage stability in the distribution network. Furthermore, through coordination between the OLTC and the energy storage system, the energy storage system's depth of discharge is significantly reduced. This allows the energy storage system to operate at a low power level, improving the system's overall voltage regulation capability, extending the cycle life of the energy storage system and ensuring optimal equipment utilization.

Claims

1. A method for coordinated voltage control of a distribution network based on OLTC and energy storage system, characterized in that: The following steps are involved: Step 1: Establish a distribution network model that includes the OLTC, energy storage system, and photovoltaic system. The load at each node is of constant power factor type, the photovoltaic system adopts maximum power point tracking control, and the energy storage system adopts the PQ control strategy. Step 2: Collect the active power and reactive power of each node in the distribution network, as well as the active power of each photovoltaic system. Calculate the node voltage of each node in the distribution network based on the distribution network power flow calculation method. Determine whether the node voltage exceeds the limit based on the node voltage. If the node voltage does not exceed the limit, repeat the node voltage calculation. If the node voltage exceeds the limit, use the communication system in the distribution network to collect the state of charge of all energy storage systems and calculate the weights and weighted voltage average of all node voltages in the distribution network. Step 3: During the OLTC control process, the voltage error is determined by the deviation between the weighted voltage average and the OLTC secondary voltage reference value, and the required tap position adjustment is calculated based on the voltage error. Step 4: The energy storage system controller includes two control modes; Voltage control mode: When the node voltage of the energy storage system is lower than the minimum node voltage or higher than the maximum node voltage, the energy storage system operates in voltage control mode; the charging and discharging active power of the energy storage system is controlled by the PI controller, and the input of the PI controller is the node voltage deviation; the state of charge limiter is used to check whether the state of charge of the energy storage system exceeds its limit, and then the voltage control mode node considering the state of charge limit is obtained. i The energy storage system charging and discharging active power: SOC control mode: When the node voltage is between the maximum and minimum limits, the energy storage system operates in SOC control mode. The charging and discharging active power of the energy storage system is controlled by the state of charge deviation. Step 5: When the energy storage system's state of charge is within the limit, the node voltage deviation is adjusted by the energy storage system's charging and discharging active power. When the energy storage system's state of charge exceeds its upper and lower limits, the node voltage level cannot be maintained by providing the energy storage system's charging and discharging active power. At this time, the node voltage deviation and state of charge deviation both increase, thereby increasing the weight of the corresponding node. The corresponding node voltage level is adjusted by changing the OLTC tap position.

2. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 1, characterized in that: The node voltage weight is calculated as follows: in, w i For nodes i The node voltage weight, ∆V i For nodes i The node voltage deviation, that is, the node i Node voltage V i With reference voltage V i，ref The difference, ∆SOC i For nodes i The state of charge deviation of the node i The state of charge of the energy storage system SOC i With node i Reference value of state of charge of energy storage system SOC i,ref The difference.

3. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 2, characterized in that: The weighted average voltage of all nodes in the distribution network is calculated as follows: Where N is the number of nodes in the distribution network.

4. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 3, characterized in that: The voltage error is calculated as follows: in, e is the voltage error, V ref is the OLTC secondary side voltage reference value, V avg is the weighted average voltage of all nodes in the distribution network.

5. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 4, characterized in that: The way to calculate the required adjustment wiper position from the voltage error is as follows: in, k Adjust the tap position as required ， V step is the voltage improvement value of each tap step of OLTC.

6. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 3, characterized in that: Determine the PI controller reference voltage based on the node voltage V PI,ref : in, V i For nodes i The node voltage, V min is the minimum node voltage, V max is the maximum node voltage.

7. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 3, characterized in that: Voltage Control Mode Node i The charging and discharging active power of the energy storage system is determined as follows: in, P i,V Node in voltage control mode i The energy storage system charging and discharging active power, P PI Output active power for PI controller; SOC min and SOC max are the minimum and maximum charge states of the energy storage system, SOC i For nodes i The state of charge of the energy storage system.

8. The method for coordinated control of distribution network voltage based on OLTC and energy storage system according to claim 3, characterized in that: Nodes in SOC control mode i The charging and discharging active power of the energy storage system is: in, P i,SOC is the charging and discharging active power of node i under SOC control mode, R is a constant, S t is the scaling factor, SOC i For nodes i The state of charge of the energy storage system, SOC i,ref For nodes i The charge state reference value of the energy storage system.

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